Multi-channel grating structures are typically written into photosensitive waveguides. The grating structure comprises a refractive index profile induced in the photosensitive waveguide, which in turn determines the optical characteristics such as the reflection, transmission, and group delay characteristics of the resulting grating structure.
The amplitude and phase of the refractive index profile can be described by a grating design function, which in turn is used in the control of an apparatus for writing the grating structure into the photosensitive waveguide. Typically, the writing apparatus comprises an optical interferometer.
It is known that a maximum refractive index contrast required to write multi-channel gratings increases with increasing number N of channels. Since photosensitive materials, such as a photosensitive fibre used to fabricate a Bragg grating, have material limits of the maximum achievable photo-induced refractive index change Δn, this represents a limitation on the maximum number of channels N that can be recorded in a given material. Thus, it is highly desirable to reduce a required ΔnN as much as possible.
At least preferred embodiments of the present invention seek to provide a method of improving a grating design function in terms of a reduced maximum refractive index variation required in the waveguide material along the grating structure, without compromising the quality of spectral characteristics.